The invention pertains to an overload-proof pressure sensor of the type described in the preamble of claim 1.
When measuring the pressure in pipelines or containers with installed valves, so-called pressure surges are caused when the valves are opened from the sudden inflow of the process fluid. The pressure peaks caused by these pressure surges are occasionally so intense that the measuring membrane of the pressure sensor may become damaged or destroyed by these surges. This means that the operability of the pressure sensor can no longer be ensured.
EP 373 536 B1 describes a pressure sensor arrangement with means for protecting the pressure sensor from overloads in the form of undesirable pressure surges. This pressure sensor contains a stepped chamber that supports the measuring membrane by means of a contact surface in case of an overload. If the pressure to be measured exceeds an upper pressure limit, it thus protects the measuring membrane from becoming damaged or destroyed.
However, the aforementioned pressure peaks not only occur when the valves are opened, but also when they are closed. The backflow of the process fluid causes undesirable negative pressure surges. The pressure peaks caused by negative pressure surges may be equally as intense [as the excessive surges]. The membrane may become damaged or even destroyed in case of such an underload.
A pressure sensor with overload and underload protection means has not been disclosed so far.
Consequently, the objective of the present invention is to make available a pressure sensor of the initially described type which is also highly underload-proof.
According to the invention, this objective is attained with a pressure sensor that is realized in accordance with the characteristics of claim 1.
The invention proposes a pressure sensor of the initially described type which is characterized by the fact that it provides a second chamber in order to protect the measuring membrane if the pressure to be measured falls short of a lower pressure limit. The second chamber is arranged between the measuring membrane and a contact plate. The contact plate is situated on the side of the measuring membrane which lies opposite the base body. A bore is provided for subjecting the measuring membrane to pressure.
This pressure sensor has an optimal resistance to pressure. It has a resistance to an excess pressure in case of an overload, as well as a resistance to a negative pressure in case of an underload.
One particularly advantageous refinement of the invention is achieved if at least one of the chambers contains an annular step. The annular contact surfaces of the respective steps formed in this way provide an optimal support effect in case of an overload or an underload, so that the measuring membrane can be additionally protected from destruction or damage. It is particularly advantageous if the chambers are outfitted with several steps.
It is preferred that the base body and the contact plate are connected to the measuring membrane by means of annular diffusion-tight connections. A solder glass connection is preferably utilized as the diffusion-tight connection. Naturally, this connection can also be done with any other connecting material that has an acceptable diffusion-preventing effect similar to glass or quartz glass. Acceptable connections may be a hard-soldered welded or bonded. However, the solder glass rings provide the advantages of being electrically insulating, and thermally stable even at high temperatures. The rings also prevent any diffusion of gases or hydrogen from the outside toward the inside.
In one preferred embodiment of the invention, the base body and/or the measuring membrane and/or the contact plate are manufactured from a ceramic material, e.g., Al2O3-ceramics, SiC-ceramics, glass ceramics, quartz or ZrO2-ceramics. However, the base body can also be manufactured from an iron-nickel alloy because it has a coefficient of thermal expansion similar to the above-mentioned ceramics. It is also particularly advantageous that the measuring membrane consists of a preferably metallic corrosion-proof material, such as special steel. When using different materials for the measuring membrane and the base body, the materials need to have a largely similar coefficient of thermal expansion to ensure that the fewest possible mechanical distortions occur during temperature fluctuations. It is particularly preferred to manufacture the base body, measuring membrane and the contact plate entirely from ceramics.
The pressure exchange medium typically consists of an oil, such as hydraulic or silicone oil. However, it would be possible to use any other fluid or even a gas.
The pressure sensor is in the advantageous form of a capacitive pressure sensor. The measuring membrane either forms the layer electrode or a circular or annular layer electrode is applied onto the measuring membrane. The layer electrode of the measuring capacitor is arranged in the chamber between the measuring membrane and the base body. A vacuum or a conventional gas with a corresponding disruptive strength is typically used as the dielectric.